A first aspect of wind turbines relates to a requirement to be able to brake the rotor blades in the case of an emergency such as a utility grid failure. Therefore, wind turbines are often equipped with aerodynamic braking systems. The aerodynamic braking system functions by rotating the rotor blades, typically into a feathered position, so that the rotor blades are prevented from taking up power from the wind, thus achieving a deceleration of the rotor speed. The braking is often combined with a mechanical braking system as well.
Thus, when the rotor has been braked, the wind turbine is not able to supply itself with any electric power unless batteries, capacitors, or diesel engine driven generators are used. Not until the grid failure has been recovered and the utility grid connection has been re-established and the wind turbine is fully operational again is the generator itself capable of generating electric power. Therefore, the essential functions of different systems of the wind turbine, such as systems for lubrication, cooling, heating, de-icing etc. will stop working or will only work as long as the batteries, capacitors or diesel engines are operational during the period of the utility grid failure, implying that the wind turbine may not be in a condition to produce electric power at the moment of the recovering of the utility grid failure, if the utility grid failure is of a certain period of time.
A second aspect of wind turbines relates to the wind turbine needing time to bring the critical components back into an operational condition before a utility grid reconnection is possible. The duration of the preparation period may vary from seconds to hours or perhaps even days, depending on factors such as the duration of the utility grid failure and the outside temperature, the humidity and the wind conditions, all of which factors influence the essential functions of the wind turbine. For instance, it may be necessary to heat up the gearbox to an operational temperature, which may take hours.
It is possible to use auxiliary power supplies during the utility grid failure in order to maintain at least some of the critical components of the wind turbine operational during the utility grid failure and thereby making the period of time required for a reconnection independent of the duration of the utility grid failure and thereby reducing to a minimum the period of time required for the reconnection. However, the period of time when the critical components can be maintained operational is often limited due the limited energy storage capacity of the auxiliary power supply, unless power supplies such as diesel engine driven generators are used.
A third aspect of wind turbines relates to the braking of the rotor during utility grid failure. According to the first aspect, i.e. a requirement to be able to brake the rotor blades in the case of an emergency such as a utility grid failure by means of aerodynamic braking systems, the aerodynamic braking must be effected very fast so that the rotational speed of the rotor will not accelerate so much that it will be difficult braking the rotor or so much that the mechanical loads to the main shaft and the bearing will be to excessive. Thus, the structural stability and strength of vital parts of the wind turbine have to be dimensioned in respect of the strong forces and high torques, which may arise during braking.
EP 1 128 064 discloses an electric pitch change device for a wind turbine comprising a back-up power unit consisting of a least one auxiliary permanent magnet generator assigned to the rotor shaft. The auxiliary permanent magnet generator delivers electrical power to electric motors, which are used to rotate the rotor blades into their feathered position in the case of an emergency, e.g. a utility grid failure. The generators are connected to the electric motors via a contact, so that when the contact is switched the rotor blades will pitch into the feathered position and thereby the rotor will be braked. Furthermore, when the rotor stops rotating, the angular position of the rotor blades will remain unchanged. Thus, EP 1 128 064 describes a method of safely braking the rotor in a robust way, but it does not provide a method for fast reconnection to the utility grid.
WO 02/44561 describes a wind turbine having an auxiliary generator for supplying electric energy from the kinetic energy of the rotor shaft. A switch device is incorporated for switching between conducting electric energy from the main generator during normal operation and conducting electric energy from the auxiliary generator during a disconnection from the utility grid. The electric energy supplied during utility grid disconnection is used for pitching the rotor blades into a flag position or feathered position, thereby braking the rotor and the main generator. WO 02/44561 describes braking of the rotor by using an auxiliary generator when the wind turbine is disconnected from the utility grid, but it does not provide a method for fast reconnection to the utility grid.
DK 174 411 published in the English language describes a method for controlling the pitch of the blades during periods of utility grid disconnection. A control is established ensuring a rotational sped of the rotor and of the generator within the normal speed range, also during disconnection from the utility grid, and where the rotational speed of the rotor and of the generator is already within the normal speed range, when the wind turbine is reconnected to the utility grid. Thereby, it is possible to obtain a fast reconnection. However, DK 174 411 does not mention any means for operating systems of the wind turbine such as the pitch control system during the disconnection from the utility grid.
U.S. Pat. No. 5,907,192 describes a wind turbine where the rotational energy which is present in the rotor and the rotor shaft is used for generating power for the pitch control system during emergency braking subsequent to the wind turbine having been disconnected from the utility grid. U.S. Pat. No. 5,907,192 describes braking of the rotor by using kinetic energy present in the rotating parts of the wind turbine when the wind turbine is disconnected from the utility grid, but it does not provide a method for fast reconnection to the utility grid.